1. Technical Field
The present invention relates generally to circularly polarizing reflective material made from single layer Cholesteric Liquid Crystal (CLC) film material having “super” broad-band reflection and transmission band characteristics approaching 2000 nm, and also to various novel methods for fabricating and using the same in diverse applications.
2. Background Art
In the modern world, there are numerous applications which require circularly polarizing material having broad-band reflection and transmission characteristics. Such applications range from polarizing filters used in optical systems, to highly reflective pigments used in the manufacture of CLC-based paints and inks.
A detailed review of the prior art literature reveals that European Patent Application 94200026.6 entitled “Cholesteric Polarizer and Manufacture Thereof”, published Jul. 20, 1994 and assigned to Philips Electronics, N.V. of Eindhoven, Netherlands (the “Phillips reference”), is the most relevant prior art reference as it discloses several methods on how to make a single layer CLC film material having broad-band reflection and transmission characteristics. In order to achieve its broad-band reflection and transmission characteristics, which are limited to about 400 nm, the Phillips disclosure requires adding a UV dye into the CLC mixture in order that the pitch of the CLC material change “linearly” from its maximum value at one film surface to its minimum value at the other film surface, wherein the difference between the maximum pitch and minimum pitch is greater than 100 nm.
According to the first fabrication technique disclosed in the Phillips reference, prior art CLC polarizing material is formed from two polymerizable chiral and nematogenic monomers, each of which has a different reactivity. During polymerization of the mixture by means of actinic radiation, a linear variation in actinic radiation intensity (i.e. a linear actinic radiation intensity gradient) is realized across the optically active layer of film by introducing an ultraviolet (UV) absorbing dye into the original mixture. This linear radiation intensity gradient causes the most reactive monomer to be preferentially incorporated into the least reactive monomer to occur at the locations of the highest radiation intensity. As a result, at least one concentration gradient of free monomers is formed during polymerization, causing the monomer to diffuse from locations with a low monomer concentration to location with a high concentration. The monomers of high reactivity diffuse to locations where the radiation intensity is highest. This diffusion process results in an increase in reactive monomers in areas of the formed polymer material where, during polymerization, the radiation intensity is highest. As a result, the composition of the material varies in a direction transverse to the surfaces of the film such that a “linear variation” in the pitch of the molecular helices results in the layer formed by the polymer. The liquid crystal material is distributed linearly across the thickness of the film. This variation in pitch provides the optically active layer with a bandwidth proportional to the variation in the pitch of the molecular helices. In thin CLC film structures, the maximum bandwidth variation achievable using this prior art fabrication technique is about 400 nm.
According to the second fabrication method disclosed in the Phillips reference, the spontaneous diffusion of monomers into a polymerizable CLC film is followed by UV polymerization. This fabrication method is carried out by depositing a film of reactive monomers on the surface of a polymerized film of CLC material. The diffusion of monomers into the CLC film layer causes a concentration gradient in the layer before diffusion is halted. As a result, the original CLC layer swells slightly causing an increase in pitch of the molecular helices. This provides a concentration gradient which, in turn, results in a “linear variation” in pitch across the film thickness. Polymerization of the layer by actinic radiation halts diffusion providing a broad-band polarizer having reflection characteristics approaching 400 nm in thin CLC film structures.
Notably, in fabrication techniques disclosed in the Phillips reference described above, the two principal materials utilized in the starting mixtures thereof are characterized as monomers having different reactivities. Moreover, when a dye is not utilized in the fabrication processes of the Phillips reference, a diffusion gradient is not established and both of the principal materials are polymerized, resulting in a narrow band polarizer.
While the above described Phillips reference discloses several methods for fabricating CLC-based circularly polarizing film having reflection characteristics approaching 400 nm in thin film structures, such bandwidth characteristics are inadequate in numerous applications where bandwidth characteristics up to five times greater are required. Also, such prior art fabrication methods require that the constituent materials both be polymerizable, restricting the many types of commercially available material that can be used during manufacture.
Thus, there is a great need in the art for circularly polarizing film material having reflection and transmission characteristics over a bandwidth that is substantially greater than the bandwidth provided by all prior art circularly polarizing material.